|A bottlenose dolphin wearing a marine basket |
sponge in Shark Bay.
By Sindya N. Bhanoo, The New York Times, July 25, 2011
In the 1980s it was discovered that some bottlenose dolphins in Shark Bay, Australia rip up marine basket sponges from the seafloor and place them on their noses for protection as they forage for food along rocky substrate. It is the only known instance of tool use by dolphins in the wild.
Now Janet Mann and Eric Patterson, biologists at Georgetown University, report that the dolphins do this because it allows them to uncover prey undiscoverable by echolocation.
Their research appears in the journal PLoS One.
“It’s well known that over 90 percent of the echo comes from the swim bladder, and a lot of benthic fish lack swim bladders,” Mr. Patterson said.
He and Dr. Mann mimicked the dolphins by placing marine sponges over poles and foraging across the seafloor.
About 65 percent of the time, they uncovered barred sand perch — fish that lack swim bladders. The dolphins may prefer sand perch because they have a higher fat content than fish with bladders.
“Not only are they providing the dolphins an extremely reliable meal, but they might also be more nutritious,” Mr. Patterson said.
Curiously, only 54 dolphins in Shark Bay use marine sponges as tools. All are female, and some pass the skill to their daughters.
When the tool use was first discovered 26 years ago, there were five dolphins that exhibited the behavior. One of those females has since died, but the other four continue to use marine sponges, Dr. Mann said.
“There’s no question it’s a lifetime commitment to the behavior,” she said. “There are three generations of spongers now that we’ve documented.”
She and Mr. Patterson would also like to understand why the skill is passed on sporadically.
“Nobody becomes a sponger without their mother being a sponger, but not all daughters becomes spongers,” she said. “One thing we’re trying to figure out is whether there are other constraints.”